1. Field of the Invention
This invention relates to an observation optical system, and in particular, to an observation optical system in which an ordinary forward observation is made and at the same time, a backward observation can also be made.
2. Description of Related Art
As a conventional observation optical system in which the forward observation and the backward observation can be made at the same time, an optical system set forth, for example, in Japanese Patent Kokai No. 2000-322564 is available. This optical system is related to an omnidirectional visual sensor and includes a wide-angle lens and a curved-surface-shaped reflecting mirror. The wide-angle lens is provided to image visual information in horizontal and upward directions. The curved-surface-shaped reflecting mirror is placed at the lower portion (on the image pickup side) of the wide-angle lens and is used to image visual information in horizontal and downward directions. The visual information obtained through the wide-angle lens and the curved-surface-shaped reflecting mirror is imaged using a common camera.